In the inductively coupled plasma mass spectrometer, the inductively coupled plasma optical or atomic emission spectrometer or the like, an inductively coupled plasma generating device is employed. In this example, the radio frequency amplifier of the inductively coupled plasma generating device will be described. FIG. 4 is a schematic diagram showing an inductively coupled plasma generating device. In the figure, reference numeral 1 denotes an amplifier. A coil 4 of a plasma generating portion is driven through a matching box 3 by the amplifier 1. As the number of turnings of the coil, for example, about 3 turns are used as shown in the figure. The matching box 3 is disposed in order to match a plasma side impedance and an output impedance of the amplifier 1, but is not always required.
The coil 4 is wound on a plasma torch 6 which is made up of triple tubes of quartz. Then, an output of the amplifier 1 is connected to one end of the coil 4 through the matching box, and the other end of the coil 4 is grounded. For example, argon (Ar) gas is made to flow into the plasma torch 6 and a radio frequency current is made to flow in the coil 4, to thereby supply a power to the plasma 5 by inductive coupling so as to maintain discharge. When plasma is ignited, a small spark discharge is allowed to be generated within the plasma torch 6 by a discharge device not shown, and a transitional impedance matching according to a proper radio frequency power to ignite the plasma and a state in which no plasma exists is taken so that the power is supplied to the plasma 5 from the coil 4 with inductive coupling by rapidly increasing the current of spark discharge, thus igniting the plasma.
In a stationary state where the plasma is ignited, when the radio frequency output is operated with, for example, 1 kW, the amplifier 1 is heated with several hundreds of W because of power loss. Therefore, a cooling pipe 2 is disposed on a heating portion of the amplifier 1 so that cooling water circulates. In this way, the amplifier 1 is so cooled as to prevent the parts from being destroyed due to a heat.
In this case, the coil 4 serves as a load of the amplifier 1. FIGS. 5A and 5B are schematic diagrams showing a radio frequency amplifier, in which FIG. 5A is the radio frequency amplifier and FIG. 5B is an equivalent circuit thereof. This circuit is an unsaturated operation power amplifier which is a transformer coupling push/pull amplifier using a MOS-FET (metal-oxidized semiconductor field effect transistor, hereinafter referred to as “FET”) as an amplifying device of A/B/C class. This example shows the use of an FET as the amplifying device, but the same circuit structure is applied even if another amplifying device is employed. An input radio frequency signal is inputted to a primary winding of a transformer T1. A radio frequency signal is generated on a secondary winding of the transformer T1 in accordance with the primary winding. A bias voltage that determines the operating point of the amplifying device is applied to a neutral point of two windings on the secondary winding of the transformer T1. A radio frequency signal resulting from synthesizing those signals is inputted to gates of FETs Q1 and Q2. The FETs Q1 and Q2 operate in accordance with the input radio frequency signal, and the functions thereof can be represented by a variable resistor VR shown in FIG. 5B. Loads of the FETs Q1 and Q2 are connected to a primary winding of a transformer T2, and a voltage V is applied to the neutral point of the primary winding.
When the FETs Q1 and Q2 operate in accordance with the input radio frequency signal, the primary winding of the transformer T2 is driven, and a radio frequency power is transmitted to a secondary winding of the transformer T2. A load 7 is driven by the radio frequency. As a result, the radio frequency power that is supplied to the load 7 can be varied in accordance with the input signal under the control.
In the above-mentioned conventional unsaturated operation power amplifier, the controllability is excellent because the output power is obtained in accordance with the input signal. However, in case of the B-class operation, there arises such a problem that the efficiency cannot be enhanced so that the theoretical maximum efficiency is 78.5%. In the case where the power amplifier is operated by, for example, an output of 1.6 kW, the efficiency is limited to about 60% in fact, and heat of 1 kW or less is generated in the amplifier. The cooling capability of the cooling water circulating device for removing the heat is increased correspondingly. A large-sized cooling water circulating device is large in noise, as a result of which because an expensive cooling water circulating device which is high in cooling capability is required and the cooling water circulating device is noisy, the cooling water circulating device may be located in another room in some cases. In addition, a power supply capacity itself of the amplifier increases, resulting in an inhibition factor of downsizing of the entire device.